Friction welding

ABSTRACT

The disclosure provides an improved method of carrying out friction welding operations in which the workpieces are heated by friction produced by rotating one in rubbing contact with the other. The rotation of the one workpiece is effected by a drive from a power source until an equilibrium temperature condition has been reached, after which the power source is disconnected and continued rotation applied to the workpiece by energy stored in a rotating mass, the energy in said mass being dissipated in completing the weld.

United States Patent [191 Hunter et al.

[ FRICTION WELDING [75] Inventors: Anthony John Hunter; Robert GrahamForbes, both of Inverness, Scotland [73] Assignee: A. I. WeldersLimited, Inverness,

Scotland [22] Filed Nov. 6, 1970 [21] Appl. No.: 87,373

[30] Foreign Application Priority Data Nov. 14, 1969 Great Britain55,895/69 [52] U.S. Cl. 29/4703, 228/2 [51] Int. Cl B231: 27/00 [58]Field of Search 228/2; 29/4703; 156/73 [5 6] References Cited UNITEDSTATES PATENTS 3,235,162 2/1966 Hollander 29/4703 June 26, 19733,417,457 12/1968 Burke et al. 29/470.3 3,455,494 7/ 1969 Stamm 29/4703X 3,595,462 7/1971 Hirayama 29/4703 X Primary Examiner-J. SpencerOverholser Assistant Examiner-Robert J. Craig Attorney-Stevens, Davis,Miller & Mosher ABSTRACT The disclosure provides an improved method ofcarrying out friction welding operations in which the workpieces areheated by friction produced by rotating one in rubbing contact with theother. The rotation of the one workpiece is effected by a drive from apower source until an equilibrium temperature condition has beenreached, after which the power source is disconnected and continuedrotation applied to the workpiece by energy stored in a rotating mass,the energy in said mass being dissipated in completing the weld.

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LIN/TS C YCLE T/ME 1 FRICTION WELDING This invention relates to theprocess for bonding workpieces together which is known as friction weld-In carrying out the friction welding process, it has been proposed toemploy power means directly to rotate one workpiece in rubbing contactwith the other to induce the required frictional heating, the drivemeans beingdisconnected when the desired plastic state of the materialhas been attained. The relative rotation is then i either rapidlystoppedor allowed to come to rest under the natural resistance createdas the interface cools .due to discontinuance of input energy.

This conventional, .or continuous drive, friction welding has beendescribed in great detail in Czechoslovakian and Russian technicalliterature dating back to 1957. It has also been described,but in lessdetail, in German patent specifications published in 1925/26 and in aBritish patent specification published in 1941.

it has also been proposed to employ a rotating inertia mass which isaccelerated to a predetermined speed by power means and to utilize theenergy stored in the inertia mass to rotate one of the workpieces andproduce the required heating thereof, the driving of the inertia massbeing discontinued before the rubbing contact of the workpiece isinitiated, so that the whole of the energy for the bonding processisprovided by the known energy stored in therotating mass.

It has further been proposed to obtain specific characteristics requiredby the materials being bonded by controlling the rotational speed on abasis of time throughout the entire weld cycle.

Each of the above-mentioned proposals has disadvantages. The firstproposal requires a relatively large driving motor and calls for arelatively long weld cycle time, as well as having an energy requirementwhich is not precisely repeatable. In the second proposal, a shorterweld cycle time can be obtained,but the energy requirement variesgreatly with the initial condition of the parts, and the final length ofa component produced by welding two workpieces together varies with theinitial length tolerances of the workpieces. Moreover, with this secondproposal, the short cycle time may result in a heat affected zonewhichrequires subsequent heat treatment, and in order to ensure a weldextending over the whole area of the mating surfaces a high inertialenergy may be required. In the third proposal initial part condition andaccuracy can affect repeatability of quality and final length tolerancesare again dependent on the initial tolerances.

It is the object of the present invention to provide a friction weldingmethod which avoids or greatly reduces the disadvantages of each of theprevious proposals whilst providing, at least in part, the advantages ofeach. It is a furtherobject of the invention to provide apparatus forcarrying out the said method.

According to one aspect of the invention there is provided a method ofbonding workpieces across a comvmon interface in which heating of theworkpieces to a plastic condition at the said interface is effected byfriction produced by rubbing engagement of the workpieces at theinterface, said method comprising mount ing one of the workpieces on arotatable member capable of being rotated by power means, and ofapplying to the workpiece energy stored in a rotatable inertial massmounting the other workpiece so that it is held against rotation,contacting the workpieces under pressure for an initial period duringwhich rotation is imparted to the workpiece mounted on the rotatablemember and energy is supplied to the said workpiece from the said powermeans, disconnecting the power means and continuing said contact underpressure for a subsequent period during which the power means remainsdisconnected and energy stored in the inertial mass is absorbed at theinterface.

Preferably the inertial mass is driven by the said power means.

Preferably the power means are disconnected when a critical condition(as herein defined) of the workpieces is attained.

According to another aspect of the invention there is provided a machinefor bonding workpieces across a common interface in which heating of theworkpieces to a plastic condition at the said interface is effected byfriction produced by rubbing engagement of the workpieces at theinterface to develop weld heat by friction and plastic working, saidmachine comprising a nonrotatable workholder for one workpiece, arotatable workholder for another workpiece, means for moving one of theworkholders longitudinally to exert pressure between the workpieces,power means for rotating said rotatable workholder, an inertial massmounted to rotate with said rotatable support, rotation transmittingmeans for transmitting drive from the power means to said rotatablesupport and to said inertial mass, and drive interrupting means to allowsaid rotatable workholder to be rotated by energy stored in saidinertial mass unassisted by said power means, means to produce a signalduring relative rotation, in rubbing contact, of the workpieces, andcontrol means for said drive interrupting means operable at theproduction of said signal to disengage the said drive.

The rotation transmitting means may incorporate a hydrostatic systemcomprising a liquid pump and motor and control means for saidhydrostatic system to provide a predetermined relative rotational speedbetween the workpieces at the commencement and/or during the part of thewelding cycle during which the said one workpiece is driven by the powermeans and to provide storage of a predetermined amount of energy in theinertial mass.

It will be understood that the method and apparatus according to theinvention are primarily concerned with the bonding together of metalworkpieces, but their use for the bonding together of workpieces ofother materials capable of being brought to a plastic condition byfrictionally-produced heating is not ex? cluded.

The method according to the invention enables the contacting surfaces ofthe workpieces to be brought to a predetermined condition during theinitial period of relative rotation so that the energy required tocomplete the bond, which energy is stored in the inertial mass, isindependent of the initial condition of the workpieces.

Since, in the method according to the present invention, the heatingphase is carried out before the stored energy begins to be utilized, andthe said heating phase is arranged to establish, by choice of rotationalspeed and axial thrust, an equilibrium interface temperature at whichthe heat input is balanced by the heat dissipation, the period of thesaid heating phase can vary with parts in a state of equilibrium withoutaffecting the period of the bonding phase and, provided that thechange-over from one phase to another occurs within this state ofequilibrium and is actuated by means, such as a limit switch, dependentfor its operation on the relative positions of the workholders the finallength of a component produced by the welding method is independent ofvariations, due to tolerances in the initial length or the preparedcondition of the contacting faces, any excess length of the workpiecebeing burnt off before the bonding phase commences and this bondingphase, utilizing an exactly repeatable ammount of stored energy, willalways commence when the parts are in'a repeatable optimum state.

If final length of'the component to be produced is not important thecritical condition may be the condition when the equilibrium interfacetemperature is attained but if final length is important, the criticalcondition includes two factors, namely the attainment of the saidequilibrium temperature and the reduction of length of the workpieces,by burning off or extrusion of material, to a predetermined length. I

Since the interface condition of the workpieces when the criticalcondition is attained is exactly repeatable, this state may be used as aweld quality monitor, the sensing device being set to indicate not onlythat the equilibrium state has been attained, but also to indicate theactual velocity of advancement of the axially movable workpiece whilstthe equilibrium state exists. If the said velocity lies outside apredetermined range of values, the sensing device is arranged to actuatesignal producing means which would indicate that the quality of the weldwas suspect. Such signal producing means are in themselves known andwill not be described in detail herein.

The invention will be hereinafter described with reference to theaccompanying drawings in which;

FIG. 1 is a side elevation of one form of friction welding machineembodying the invention;

FIG. 2 is a side elevation of the machine shown in FIG. 1 with partsbroken away;

FIG. 3 is a side elevation, similar to FIG. 1, of another form offriction welding machine embodying the invention;

FIG. 4 is a side elevation of the machine shown in FIG. 3, withparts'broken away;

FIG. 5 is a side elevation, similar to FIG. 1, of another form offriction welding machine embodying the invention;

FIG. 6 is a side elevation of the machine shown in FIG. 5, with partsbroken away; and

FIGS. 7 to 12 inclusive are graphs showing typical curves of rotationalshaft speed and axial velocity of workpiece movement plotted againsttime in friction welding cycles used when carrying out the methodaccording to the invention. The curves of rotational shaft speed areincompletely shown, representating only the axial velocity up to thepoint at which rotation ceases.

In the friction welding machines shown in FIGS. 1 and 2 and in FIGS. 3and 4, the rotating workpiece rotates about a horizontal axis, and inthefriction welding machine shown in FIGS. 5 and 6 the rotating workpiecerotates about a vertical axis.

In the machine shown in FIGS. 1 and 2 the machine comprises a hollowbase 12 on which is mounted a housing 13 enclosing a slidable headstock14v (FIG. 2)

carrying a rotatable workholder 15, the housing also enclosing othercomponents hereinafter described. Also mounted on the base 12 is atailstock 16 carrying a fixed workholder 17, the workholders 15 and 17being positioned to support two workpieces such as are shown at 18 and19 in FIG. 2, in co-axial relation one to the other. r Referring to FIG.2, the headstock 14 is slidably guided on the base 12 by co-operatingguide means 21, 22 .for movement towards and away from the tailstock 16,fluid pressure cylinders of which one is shown at 23, being provided tomove the headstock in the direction in which it is guided. The cylinders23 are anchored at 24 to the frame 12 and have slidable in them pistonsconnected by rods 25to the headstock 14. The workholder 15 is mounted ona shaft 26 rotatably mounted in the headstock 14 and arranged tosupport, at its end remote from the workholder 15, an inertial mass 27which at all times rotates with the shaft 26. As shown, the inertialmass consists of a plurality of discs 28 clamped to a flange 2 fixed tothe shaft 26, and a fixed shaft 31, co-axial 'with the shaft 26, ismounted in the housing 13, discs 28 being shiftable by sliding from oneshaft to the other so that any selected number of discs can be mountedon and clamped to the shaft 26 depending onthe value of the inertialmass required, any unwanted discs being supported on thefixed shaft 31.

A hydrostatic drive is provided for the shaft 26, the said drivecomprising a liquid pump 32-mounted in the machine base 12 and driven byan electric motor 33, and a liquid pressure motor 34 mounted on theheadstock 14 and connected to the pump 32 by flexible conduits 35, theoutput shaft of the motor 34 being coupled to the shaft 26 by drivemeans 36 such as a toothed belt and pulleys, a chain-and-sprocket gearor spur gearing. The hydraulic motor may be of either the radial pistontype or of the swashplate type with pistons parallel to its axis.

The pump/motor system 32, 34 is controllable in known manner so that thespeed of rotation of the motor output shaft can be varied over a widerange and the motor 34 can rotate idly without being driven by liquidfrom the pump, thus allowing the inertial mass 27 to be rotated bykinetic energy stored therein and to inpart such energy to theworkpieces.

Electrical switching means of any suitable type for starting theelectric motor and for initiating a welding cycle when the electricmotor is already running, are operated by suitable controls on a panel37, FIG. 1, which also carries controls for presetting for automaticoperation during a welding cycle conventional speedcontrolling means forthe liquid pressure motor 34 and conventional means for determining thethrust exerted on the headstock 14 by the fluid pressure cylinder 23.The electric motor is allowed to run continuously during any period forwhich the machine is in continuous use to carry out a plurality ofwelding cycles. Automatic means of any known type are provided forsetting the hydrostatic drive to the condition in which the motorrotates freely. The said automatic means are controlled to determine thepoint in a welding cycle at which the drive to the shaft 26 and inertiamass is discontinued by electrically controlled means controlled as willnow be described with reference to FIG. 2.

The said point, hereinafter referred to as the critical point isdetermined essentially by one criterion, namely the equilibriumcondition at which the heat input to the workpieces is equal to the heatdissipated,

and preferably also by a second criterion, namely the reduction of thelength of the workpieces due to burnoff and extrusion, to apredetermined length, determination by the first criterion aloneensuring uniform and repeatable weld quality and determination by bothcriteria together ensuring also constant length of the final productregardless of length variations in the initial workpieces.

An indication of the first-mentioned condition, namely the equality ofheat dissipation with heat input, is given by the fact that when thatcondition is reached, the headstock 14 advances at a constant speed andthere is therefore mounted to move with the tailstock 16 a sensingdevice 38 consisting of a speed measuring device arranged to provide asignal under constant speed conditions or an accelerometer arranged toprovide a signal under conditions of no acceleration. The said signal isprovided by the closing of an electric switch. The sensing device 38 isprovided with an operating member 39 co-operating with an adjustablestop 41 carried by the headstock 14 the stop 41 coming into engagementwith the operating member 39 substantially at the same time as theworkpieces l8 and 19 make initial contact, and the subsequent movementof the operating member controlling the sensing device. The stop 41 ispreferably provided with a micrometer adjustment.

When the constant speed condition is reached, the torque exerted torotate the rotatable workpiece is also substantially constant, so atorque meter may be provided as an alternative means of indicating thiscondition.

An indication of the second-mentioned condition, namely the burn-off orextrusion of material to leave the workpieces at a predetermined lengthis given by the arrival of the headstock 14 at a predetermined distancefrom the tailstock, and there is therefore mounted on the headstock ortailstock a limit switch 42, shown in FIG. 2 as being mounted on theheadstock 14, cooperating with an adjustable stop 43 on the tailstock,the stop 43 having a micrometer adjustment and being adapted to contactand operate the limit switch when the headstock reaches a predeterminedposition relative to the tailstock.

The two switches are arranged in series in an electrical circuit theclosing of which operates the automatic means for setting thehydrostatic drive to the condition in which the motor 34 rotates freely.

The operation of the machine described with reference to FIGS. 1 and 2to effect a welding operation is as follows.

The workpieces l8 and 19 to be welded together are mounted in thework-holding devices 15 and 17 respectively with their ends remote fromthe ends to be welded together in fixed longitudinal positions. Assumingthat the electric motor 33 is running and driving the pump 32, operationof a manual control on the panel 37 actuates a valve to initiate thetransfer of liquid under pressure from the pump 32 to the motor 34,driving the motor 34 to rotate the shaft 26, the workholder l5 and theinertial mass 27. The workpiece 18 is thus accelerated up to the desiredspeed, determined by the setting of the hydrostatic drive, and theflywheel is accelerated to the same speed at the same time by the samedrive. Liquid under pressure is also admitted to the cylinder 23 toadvance the headstock l4 and thus bring the workpieces into contact onewith the other and bring the stop 41 into contact with the operatingmember 39 of the sensing device 38. The pressure acting in the cylinder23 is preset to provide any desired pressure at the interface betweenthe workpieces.

The peak energy requirement in a friction welding operation that must bemet by input power occurs when the parts come into contact in the coldstate, i.e., during initial heating, and, since the inertial mass isdirectly connected with both the drive motor and the rotating workpiece,the kinetic energy attained by the said inertial mass is available, asin the normal application of flywheels, for damping and stabilizing thedrive. When the workpieces have been heated, by the friction betweenthem, to the plastic state, and have reached the equilibrium condition,the energy absorbed, and therefore the speed of rotation and theinterfacial condition, will have stabilized, and the inertial mass willbe rotating freely with the workpiece 18, which is driven only by themotor 34. This equilibrium condition can be continued as long as isdesired, the electric switch controlled by the sensing device 38 beingclosed when the condition is attained and remaining closed.

The said equilibrium condition will, in fact, continue until sufficientburn-off of the workpieces has taken place to allow the adjustable stop43 to contact and close the limit switch 43, the completion of theelectric circuit through the two switches actuating a valve to place thepower supply to the motor 34 in a neutral condition in which the saidmotor rotates freely and is not driven by liquid from the pump 32. Therotating parts of the said motor, with the inertial mass 27, the shaft26, the workholder l5 and the workpiece 18 thus continue to rotate dueto the kinetic energy stored in the said parts until the said storedkinetic energy is absorbed.

Since the condition of the workpieces and the amount of stored kineticenergy at the instant at which the drive to the motor 34 is stopped areprecisely repeatable for any given workpieces it follows that formationof the bond may now take place at as near optimum conditions as it ispossible to attain, and welds will be repeatable in both quality andaccuracy.

The value of the inertial mass can as above described be changed to meetthe desired conditions for any particular welding operation, includingthe degree of plastic working of the material after heating and beforethe bond is finally formed.

In the friction welding machine shown in FIGS. 3 and 4, a rigid frame 45supports, in a housing 46 fixed to the said frame, a fixed headstock 47,and a tailstock 48 slidable on guide rods 49 extending between thehousing 46 and a rigid support 51 on the frame, is movable towards theheadstock by liquid pressure acting in cylinders 52 mounted on thesupport 51 on pistons carried by rods 53 attached to the tailstock 48.The cylinders 52 are enclosed by a housing 54 shown in FIG. 3.

The headstock 47 supports a rotatable shaft 55 on which is mounted aworkholder 56, and a second workholder 57 is non-rotatably mounted onthe tailstock 48.

An electric motor 58 mounted in the frame 45 is coupled by transmissionmeans such as a belt 59 to the driving member 61 of a clutch 62 mountedon the shaft 55, the said clutch driving member 61 being free to rotateon the shaft 55 except when engaged with a clutch driven member 63rigidly mounted on the shaft.

The driving member 61 of the clutch 62 is provided with means formounting thereon discs 64, similar to the discs 28 described withreference to FIGS. 1 and 2, to provide an inertial mass 65 rotating withthe said clutch driving member, and a fixed shaft 66, similar to theshaft 31 in FIG. 2, is provided to receive any of the discs 64 not usedas part of the inertial mass 65 in any particular welding operation.

The guide bars 49, being rigidly attached to the housing 46 and to thesupport 51, support the reaction to the thrust exerted by the cylinders52 in a closed loop system formed by the housing 46, support 51 andguide bars 49, none of the forces due to axial loading being transferredto the machine frame.

A sensing device 67, corresponding to the sensing device 38 describedwith reference to FIGS. 1 and 2, is mounted in the housing 46, theoperating member 68 of the said sensing device co-operating with anadjustable stop 69 on the tailstock, also as previously described. Tocarry out a welding operation with the machine shown in FIGS. 3 and 4,workpieces 70 and 71 are mounted in the workholders 56 and 57respectively. The electric motor 58 is started up with the clutch 62disengaged and the inertial mass is brought up to a desired speed theworkholder 56 remaining stationary, so that the workpieces can be loadedwhilst the motor is running if desired.

Operation of a cycle initiation control on a control panel 72 (FIG. 3)actuates suitable control means to engage the clutch 62 and therebyproduce rotation of the workpiece 70, and to supply liquid underpressure to the cylinders 52 to move the tailstock 48 towards theheadstock 47. The workpieces 70 and 71 are therefore brought intorubbing contact one with the other, and continue to move towards eachother as they are heated and rendered plastic until the equilibriumcondition described with reference to FIGS. 1 and 2 is attained.

A limit switch (not shown), similar to the switch shown in FIGS. 1 and2, or other means responding to other chosen criteria in relation to theweld, is also provided, the said limit switch or other means acting,after the equilibrium condition has been attained, to operate meansdisconnecting the supply of electricity to the electric motor 58, sothat the workpiece 70 continues to be rotated by the kinetic energystored in the inertial mass 65, the rotating parts of the electric motor58 and the transmission means 59. Thus, as in the previously describedmachine, the workpieces are brought to an equilibrium condition whilstrotation is being imparted to the workpiece 70 by the electric motor 58and the inertial mass 65 is rotating freely, and a known and repeatableamount of stored energy is available for dissipation during the bondingphase of the welding operation. On completion of the bond between theworkpieces, when rotation has ceased due to the stored energy beingabsorbed, the clutch 62 is disengaged and the electric motor 58 isre-energized, thus bringing the ,inertial mass 65 back to the desiredspeed whilst allowing the welded workpieces to be removed and freshworkpieces to be inserted.

In the embodiment of the invention shown in FIGS.' and 6 of thedrawings, the friction welding machine is of the kind in which therotatable workpiece holder rotates about a vertical axis, and the saidrotatable workpiece holder is driven by an electric motor through aclutch instead of through a hydrostatic drive, the clutch arrangementbeing different from that described with reference to FIGS. 3 and 4.

The machine comprises a hollow pillar 74 from one side of which extendsa bare platform 75 and at the top of which is mounted a housing 76extending laterally over the platform 75. A headstock 77 is rigidlymounted in the housing 76 to support a rotatable spindle 78 held againstaxial movement, the spindle 78 having its axis vertical and carrying, atits lower end, a workholder 79. Slidably mounted on guide rods 81extending between the platform 75 and the housing 76 is a tailstock 82carrying a second workholder 83 co-axial with the workholder 79 andfixed against rotation. Fluid pressure cylinders 84 mounted on theplatform 75 have pistons acting through rods 85 on the tailstock 82 tourge the lattertowards the headstock 77.

An electric motor 86 mounted in the housing 76 drives the spindle 78through a'disengageable clutch 87, and a belt or equivalent drive 88.

An inertial mass 89 is mounted on the spindle 78 to rotate therewith,the said mass 89 consisting, as in the machine described with referenceto FIGS. 1 and 2, of a plurality of separate discs 91, and a support 92in the form of a rod of circular cross section, curved so that its endsare at right angles one to the other and fixed at one end-to a wall ofthe housing 76 has its other end co-axial with the spindle 78, so thatdiscs 91 may be shifted from the spindle 78 to the support 92 and viceversa, enabling the value of the mass 89 to be varied.

Two workpieces between which a weld is to be formed are shown at 94 andrespectively, the workpiece 94 being clamped in the workholder 79 so asto be rotatable by the spindle 78, and the workpiece 95 being clamped inthe workholder 83 so as to be held against rotation.

The guide rods 81 being rigidly connected to the platform 75 and housing76, support the reaction forces due to upward thrust exerted on thetailstock 82 by pressure in the cylinders 84 thus providing a closedloop system and avoiding the transference of forces due to axial loadingto the machine frame.

It will be observed that, in the arrangement shown in FIGS. 5 and 6, theinertial mass 89 rotates at all times with the spindle 78, the clutch 87disconnecting the electric motor from both the shaft 78 and the inertialmass 89.

A sensing device 92, corresponding to the sensing devices of thepreviously described embodiments of the invention is mounted on thepillar 71 and has an operating member 93 co-operating with an adjustablestop 94 carried by the tailstock 82.

The machine described with reference to FIGS. 5 and 6 operates in thesame manner as that described with reference to FIGS. 1 and 2, releaseof the clutch 87 having the same effect as the placing of thehydrostatic drive of FIGS. 1 and 2 in the neutral condition.

A limit switch (not shown) similar to that described with reference toFIGS. 1 and 2, or other means responding to other chosen criteria inrelation to the weld, is also provided to determine the actual point,

after equilibrium conditions have been reached, at which the clutch 87is disengaged.

As already described herein, the detection of the state of equilibriumof the workpieces is effected by monitoring the velocity of the movingworkpiece. As the axially moving workholder carries a part, underpressure, into engagement with a stationary part, the rate of axialadvancement, being directly related to the rate of burn-off of materialfrom these parts, will reflect the condition of the interface betweentheparts. When the interface is in an equilibrium condition, the rate ofburn-off and therefore the rate of axial advancement, will be constant.i

The cycle of a welding operation carried out according to the method ofthe invention may be illustrated in simplified diagramforrn by plottingrotational speed of the rotatable workpiece, and axial velocity of theaxially movable workpiece, against cycle time, and FIGS. 7 to 12inclusive are diagrams so plotted, FIG. 7 showing the basic cycle andFIGS. 8 to 12 showing variations thereof. In these Figures, the point intime at which the equilibrium state of the workpieces is reached, isindicated at PE, and the critical point at which the power drive to therotating workpiece is cut off, is indicated at PI. The period betweenthe pointsPE and PI, during which the equilibrium state is maintained isshown at Referring to FIG. 7, the rotational speed is held constant, orsubstantially constant during the period for which the power drive ismaintained in operation, the speed being affected only by the initialcontact pressure being applied and by inaccuracies in the preparation ofthe contact surfaces of the workpieces. The axial velocity also dependson the interfacial conditions during the heating up of the material atthe interfaces, and tends generally to increase up to the point PE wherethe equilibrium state is attained. After the point PE has been passed,the axial velocity remains constant until the sensing device operates,at the point PI, and the power drive ceases. Since the rotatingworkpiece is now driven only by energy stored in the inertial mass, andparts rotating therewith, the rotational speed decreases to zero duringa period when plastic working of the material takes place and the axialvelocity increases due to such plastic working, reaching a maximum at orabout the point at which relative rotation of the workpieces ceases andthen dropping to zero. The axial velocity curves in FIGS. 7 to 12 arenot extended beyond the point at which rotation ceases. The shape of thecurves beyond this point depends on a large number of variables.

The period SE can be varied, as already described, to allow a degree ofburn-off, after the equilibrium condition is reached, to bring theworkpieces to a pre-chosen length, FIG. 8 showing a cycle in which theperiod SE is extended as compared with that in FIG. 7.

The extent of the hot plastic working may be controlled by varying theinertial mass, for example by changing the number of discs included insaid mass as hereinbefore described, FIG. 9 showing the resulting curveswhen the inertial mass is increased as compared with that employed toprovide the curve of FIG. 7.

In the welding of some metals, it is advantageous to vary the rate ofheat input, and the effect of utilizing increasing or decreasing speedduring the part of the cycle when the rotating workpiece is driven bythe power source and is being heated up to the equilibrium condition isshown in FIGS. 10 andll, which may be compared with FIG. 7. Startingwith a highrotational speed causes faster heat build-up with higherlubricity, and a narrower heat-affected zone in the workpieces, whilststarting with a slow rotational speed causes lower temperaturesresulting in lower lubricity and a wider heat-affected zone for the sameaxial pressure.

Speed variation can be effected by incorporating an electro-mechanicalpressure control device or other pressure control device for thehydrostatic drive system when such a drive system is used, and thepressure control device may be used to provide a braking effect on theshaft which carries the rotatable workholder whilst it is being drivenby the stored energy, which braking effect is accurately controllable soas to provide repeatable speed variation conditions. A cycle duringwhich such braking is employed is shown in FIG. 12, and com parison ofthat Figure with FIG. 7 shows that the plastic working and bonding phaseis shortened by the braking effect.

In some cases, particularly when welding non-ferrous materials, theplastic working must be limited in its duration to allow consolidationof the newly formed bond. The subtractive effect of the braking may bein discrete steps or progressive, supplying a repeatable retardation tothe rotational speed, and therefore the stored energy of the inertialmass.

When a drive that does not provide variable speed is used, such as aconstant speed electric motor as described with reference to FIGS. 3 and4, the results shown in FIG. 12 may be obtained by incorporation ofother devices such as mechanical or electrical braking devices.

The supply of fluid pressure to the cylinders 23 (FIG. 2), 52 (FIG. 4)or 84 (FIG. 6) may be controlled to provide a constant axial thrustthroughout the welding cycle or an axial thrust which is varied in stepsor progressively during the cycle, the variation of thrust beingeffected automatically by preset control means. Such control means arewell known and will not be described herein.

We claim:

l. A method of bonding workpieces across a common interface in whichheating of the workpieces to a plactic condition at the said interfaceis effected by friction produced by rubbing engagement of the workpiecesat the interface, said method comprising the steps of: mounting a firstworkpiece on a rotatable member capable of being rotated bydisconnectable power means to apply energy from said power means to theworkpiece and when the power means is disconnected capable of applyingto the workpiece energy stored in a rotatable inertial mass; mounting asecond workpiece so that it is held against rotation; contacting theworkpieces under pressure for an initial periodduring which rotation isimparted to the workpiece mounted on the rotatable member and energy issupplied to said first workpiece from said power means; choosing thecontacting pressure and speed at which said first workpiece is driven toprovide a rate of heat generation which, when a predeterminedtemperature is reached, is equal to the rate of heat dissipation, sothat an equilibrium interface temperature condition is established;disconnecting the power means after the attainment of said predeterminedinterface temperature condition; and continuing said contact undersimilar or higher pressure for a subsequent period during which thepower means remains disconnected and energy stored in the inertial massis absorbed at the interface.

2. A method of bonding workpieces according to claim 1, wherein theinertial mass is driven by said power means.

3. A method of bonding workpieces according to claim 1, wherein thepower means is disconnected said power means when the measured speed ofadvance is substantially constant and the predetermined spacing betweenthe workholders is attained.

6. A method of bonding workpieces according to claim 3, including thefurther steps of measuring the speed of advance of one workpiece towardsthe other and disconnecting the power means when the speed of advance issubstantially constant.

7. A method of bonding workpieces according to claim 3, wherein thepower means is disconnected by the operation of a timer which isinitiated when axial pressure is exerted between the workpieces at theinterface.

1. A method of bonding workpieces across a common interface in whichheating of the workpieces to a plactic condition at the said interfaceis effected by friction produced by rubbing engagement of the workpiecesat the interface, said method comprising the steps of: mounting a firstworkpiece on a rotatable member capable of being rotated bydisconnectable power means to apply energy from said power means to theworkpiece and when the power means is disconnected capable of applyingto the workpiece energy stored in a rotatable inertial mass; mounting asecond workpiece so that it is held against rotation; contacting theworkpieces under pressure for an initial period during which rotation isimparted to the workpiece mounted on the rotatable member and energy issupplied to said first workpiece from said power means; choosing thecontacting pressure and speed at which said first workpiece is driven toprovide a rate of heat generation which, when a predeterminedtemperature is reached, is equal to the rate of heat dissipation, sothat an equilibrium interface temperature condition is established;disconnecting the power means after the attainment of said predeterminedinterface temperature condition; and continuing said contact undersimilar or higher pressure for a subsequent period during which thepower means remains disconnected and energy stored in the inertial massis absorbed at the interface.
 2. A method of bonding workpiecesaccording to claim 1, wherein the inertial mass is driven by said powermeans.
 3. A method of bonding workpieces according to claim 1, whereinthe power means is disconnected when the equilibrium interfacetemperature has been reached.
 4. A method of bonding workpiecesaccording to claim 1, wherein the power means is disconnected only whenboth the equilibrium interface temperature of the workpieces and apredetermined spacing between workholders mounting the workpieces havebeen attained.
 5. A method of bonding workpieces according to claim 4,wherein the step of disconnecting the power means further includesmeasuring the speed of advance of one workpiece towards the other andthe attainment of a predetermined spacing between workholders mountiNgthe workpieces, and thereafter disconnecting said power means when themeasured speed of advance is substantially constant and thepredetermined spacing between the workholders is attained.
 6. A methodof bonding workpieces according to claim 3, including the further stepsof measuring the speed of advance of one workpiece towards the other anddisconnecting the power means when the speed of advance is substantiallyconstant.
 7. A method of bonding workpieces according to claim 3,wherein the power means is disconnected by the operation of a timerwhich is initiated when axial pressure is exerted between the workpiecesat the interface.